The present invention pertains to inkjet ink and more particularly to an aqueous inkjet ink comprising pigment stabilized with anionic dispersant, and having cations present in the ink which comprise a certain mixture, in a certain molar ratio, of lithium and one or more of sodium, potassium, rubidium and/or cesium. The inks exhibit greatly extended latency.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor. Inkjet printers offer low cost, high quality printing and have become a popular alternative to other types of printers.
An ink-jet ink is characterized by a number of necessary properties, including color, jetability, decap time (latency), drying time and shelf-life, among others. There is, however, often a tradeoff between these properties because improving one property can result in the deterioration of another property.
The decap time of the ink is the amount of time a printhead can be left uncapped and idle and still fire a drop properly—that is to say without misdirection, loss of color or unacceptable decrease of velocity. Decap is sometimes referred to in the art as “latency” and these two terms will be used interchangeably.
Because not all the nozzles of the printhead print all the time, a printer service routine requires the idle nozzles to discharge (“spit”) on a regular basis into the waste container (“spittoon”) to avoid printing defects. It is desirable, however, to service the printhead as infrequently as possible as it is wasteful of ink and slows print speeds. To reduce need for servicing, an ink will preferably have a long decap time.
Contributing to decap problems is the trend for printheads to fire smaller drops to increase image resolution. The increased surface area to volume to the smaller drops allows faster evaporation of volatile vehicle components at the nozzle face and thereby tends to decrease decap time.
Both soluble (dye) and insoluble (pigment) colorants have been used in inkjet inks and both have certain advantages. Pigments are advantageous because they tend to provide more water-fast and light-fast images than dye inks.
In aqueous inkjet ink formulations, pigments are in the form of finely divided particles which must be stabilized to dispersion in the ink vehicle. Stabilization of the pigment particles can be accomplished by treatment with a dispersing agent, such as a surfactant or polymeric dispersant. Alternatively, pigment particles can be stabilized by surface modification to form a so-called “self-dispersible” or “self-dispersing” pigment which, as the name implies, are stable to dispersion without a dispersing agent. Treatment of pigments with dispersant tends to be the more universally applicable method of stabilization.
U.S. Pat. No. 4,597,794 discloses aqueous inkjet inks with pigment and anionic polymeric dispersant. Representative counterions include alkali metals, such as Na and K, and aliphatic and alcoholic amines.
U.S. Pat. No. 6,143,807 discloses aqueous inkjet ink comprising pigment and a dispersant which is a reaction product of an aldehyde and a naphthalene sulfonate salt. Counter-ions for the dispersant salt include Rb, Cs, K, Na, Li, substituted and unsubstituted ammonium. The ink has a latency more than 10 seconds in a high resolution printhead.
U.S. Pat. Nos. 5,085,698 and 5,519,085 disclose aqueous inkjet ink formulations with block copolymer dispersants. The dispersants are neutralized with alkanolamines and alkali metal hydroxide.
U.S. Pat. No. 6,866,379 discloses aqueous inkjet ink comprising pigment, dispersed with an oleoyl methyl taurine salt dispersant, and a water-reducible addition polymer neutralized by an alkaline metal hydroxide. In one example, the dispersant was in the potassium salt form and the polymer was neutralized with lithium hydroxide. However, the molar ratio of lithium to potassium is outside range of the present invention.
Although current pigmented inkjet inks are being successfully jetted, there is still a need in the art for, and it is an object of this invention to provide, pigmented inkjet ink with longer decap time that still retains other beneficial print properties.